The present invention relates to a hydraulic control apparatus for a vehicle with a belt-drive continuously variable transmission (CVT).
FIG. 4A illustrates a hydraulic control apparatus of a related art. As illustrated in FIG. 4A, primary pulley 300 of a belt-drive CVT includes fixed disk 301 making a unitary rotation with input rotation transmitted from an engine, and moveable disk 302 axially moveable corresponding to an oil pressure supplied to cylinder chamber 303. Transmission control valve 100 includes spool 101, port 102 communicated with a pressure regulator valve, not shown, port 103 communicated with cylinder chamber 303, and drain port 104 through which an oil pressure in cylinder chamber 303 is drained when communicated with port 103. Spool 101 is connected with stepping motor 200 via link 400. Link 400 is coupled to stepping motor 200 at one end thereof and an outer circumferential periphery of moveable disk 302 at an opposite end thereof. Thus, spool is also connected with moveable disk 302 via link 400. As moveable disk 302 moves, spool 101 is displaced. Stepping motor 200, spool 101 of transmission control valve 100, moveable disk 302 of primary pulley 300 and link 400 constitute a mechanical feedback mechanism for controlling the movement of spool 101.
Referring to FIGS. 4A and 4B, an operation of the mechanical feedback mechanism upon changing a transmission ratio of the belt-drive CVT to the Low speed side, namely, upon increasing the transmission ratio, is explained. Here, in other words, the transmission ratio of the belt-drive CVT is a pulley speed ratio between a rotational speed of primary pulley 300 and a rotational speed of a secondary pulley, not shown. FIG. 4B shows, at upper, middle and lower parts thereof, relationships between stepping motor 200, spool 101 of transmission control valve 100 and the pulley speed ratio in an initial state of the mechanical feedback mechanism, in a driven state of stepping motor 200, and in a transmission ratio change completed state, respectively. In the initial state, spool 101 is in a neutral position where the communication between ports 102, 103 and 104 are blocked. When a drive command for controlling the transmission ratio to the Low speed side is outputted to stepping motor 200 as shown in the middle part of FIG. 4B, stepping motor 200 drives spool 101 to upward move from the neutral position shown in the upper part of FIG. 4B to a drain position shown in the middle part of FIG. 4B. In the drain position of spool 101, port 103 is communicated with port 104 to thereby allow drain of the oil within cylinder chamber 303 from port 104. Owing to the drain of the oil within cylinder chamber 303, moveable disk 302 is moved downward as indicated by arrow in FIG. 4A. This causes the opposite end portion of link 400 which is coupled with moveable disk 302 to move downward. As a result, spool 101 is downward moved and return to the neutral position as shown in the lower part of FIG. 4B. In the neutral position, the communication between ports 103 and 104 are prevented so that drain of the oil pressure is stopped. The change of the transmission ratio to the Low speed side is thus completed. Japanese Patent Application First Publication No. 09-032898 discloses a hydraulic control apparatus having such a mechanical feedback mechanism.